Filtering devices that are percutaneously placed in blood vessels have been available for many years. A need for filtering devices can arise, for example, in trauma patients, orthopedic surgery patients, neurosurgery patients, or in patients having medical conditions requiring bed rest or non-movement. During such medical conditions, the need for filtering devices arises where there is a likelihood of thrombosis in the peripheral vasculature of patients wherein clot material, stenosis material or other particles break away from the vessel wall, risking downstream blockage of the vessel or other damage. For example, depending on the size, such break-away material could pose a serious risk of pulmonary embolism, i.e. wherein blood clots migrate from the peripheral vasculature through the heart and into the lungs.
A filtering device can be deployed in the vasculature of a patient when, for example, anticoagulant therapy is contraindicated or has failed. Typically, filtering devices are permanent implants, each of which remains implanted in the patient for life, even though the condition or medical problem that required the device has passed. In more recent years, filters have been used or considered in preoperative patients and in patients predisposed to thrombosis which places the patient at risk for embolism.
The benefits of a vascular filter have been well established, but improvements may be made. For example, filters generally have not been considered removable from a patient due to the likelihood of endotheliosis of the filter or fibrous reaction matter adherent to the endothelium during treatment. After deployment of a filter in a patient, proliferating intimal cells begin to accumulate around the filter struts which contact the wall of the vessel. After a length of time, such ingrowth may prevent removal of the filter, or may risk significant trauma during removal through a layer of endothelium, requiring the filter to remain in the patient. Where removal has been considered, filters have been provided with a central hook for gripping in order to pull the filter during removal. Such hooks can be difficult to find or latch onto, and may be covered by cellular growth. They further provide the opportunity only to pull the filter in one direction or to remove it in only one direction. As a result, there has been a need for an effective filter that can be removed after the underlying medical condition has passed.
Another difficulty with conventional filters is that they may become off-centered or tilted with respect to the hub of the filter and the longitudinal axis of the vessel in which it has been inserted. As a result, the filter including the hub and the retrieval hook engage the vessel wall along their lengths and potentially become endothelialized therein. In the event of this occurrence, there is a greater likelihood of endotheliosis of the filter to the blood vessel along a substantial length of the filter wire. As a result, the filter becomes a permanent implant in a shorter time period than otherwise. Moreover, a tilted filter may not provide protection against traveling emboli across as much of the cross-sectional area of the vessel as a properly-oriented filter would.
It has been a challenge to design a vascular filter with features that permit easy insertion and retrieval of the filter while maintaining the effectiveness of the filter.